Solar cells are providing widespread benefits to society by converting essentially unlimited amounts of solar energy into useable electrical power. As their use increases, certain economic factors become important, such as high-volume manufacturing and efficiency.
With reference to the schematic, cross-sectional views of the exemplary solar cells of FIGS. 1-2, solar radiation is assumed to preferentially illuminate one surface of a solar cell, usually referred to as the front side. In order to achieve a high energy conversion efficiency of incident photons into electric energy, an efficient absorption of photons within a silicon wafer is important. This can be achieved by a good surface texturing and antireflection coating on the front side and a low parasitic absorption within all layers except the wafer itself. An important parameter for high solar cell efficiency is the shading of the front surface by metal electrodes. In general, the optimized metal grid is a tradeoff of losses between shading and electrical resistance of the metal structure. The optimization for efficiency of the solar cell requires a grid with very fine fingers and short distances in between those fingers, which should have a high electrical conductivity. A practical technique for forming this structure is the subject of this invention.
Some solar cell production technologies may use screen printing technology to print the electrode on the front surface. A silver paste may be printed on top of a silicon nitride antireflection coating and fired through the coating in a high temperature process. This is a short process sequence and has therefore gained the highest market share in crystalline silicon solar cell technology. However, certain inherent properties of this approach include a comparatively broad line width in excess of 50 um, typically about 100 um, and a fairly low line conductivity of the metal grid due to the use of several non-metallic components in the printed paste. Also, the firing process results in a penetration of the metal paste ingredients through the antireflection layer into the substrate where increased recombination occurs. This holds for both cases of a front junction device where the pn junction can be severely damaged by unwanted penetration of the space charge region as well as for back junction devices where the front surface recombination is increased and significantly reduces the collection efficiency of the back junction emitter.
An improved structure for the front side metallization is shown in FIG. 1—an optimized front contact structure for a high-efficiency solar cell. The antireflection dielectric coating 2 covers the substrate 1 on the entire surface except underneath the metal contact 4. The line width of the metallization line 4 is on the order of 50 um or less and the total surface coverage with metal of the front side is about 7% or less.
The thin metal contact 4 can subsequently be plated 4′ to the required thickness in order to obtain a higher conductivity. FIG. 2 shows that the metal contact line 4 may be used as a seed layer to start plating of the electrode 4′ to a desired thickness. Using electroplating for the buildup of the line conductivity, a sufficient thickness of the metal layer 4 on the order of ˜50-500 nm is required in order to enable good plated metal 4′ uniformity. When plating is performed the antireflection coating 2 must also function as a plating barrier to prevent metal plating onto the surface of the substrate, and for this reason alone the antireflection coating must be a good electrical insulator, e.g. a largely intact dielectric film.
State of the art technologies for the formation of conductive metal grids on solar cells are too expensive and/or have specific performance limitations. A simple, high performance and cost effective means for the formation of conductive metal grids on solar cells is therefore required.